In recent years, a study of a microphone array technology for (1) emphasizing signals coming from a specific direction, (2) estimating a direction from which the signals come when the direction is unknown, and (3) separating a plurality of sound sources coming from the different directions using a plurality of microphones is becoming active.
A first method of this technology is a most simple method of a microphone array, and Delayed Sum Array is exemplified (see “Computer-steered microphone arrays for sound transduction in large rooms,” by J. L. Flanagan, J. D. Johnston, R. Zahn, and G. W. Elko, J. Acoust. Soc. Am., vol. 78, No. 5, pp. 1508-1518, 1985). In this method, signals from a specific direction are emphasized, that is, a directional property is formed in the specific direction on the basis of a principle such that when a predetermined delay is inserted into signals from respective microphones and adding these signals, only signals coming from a preset direction are combined and emphasized in the same phase, while signals coming from other directions are not aligned in phase and hence are mutually attenuated.
As a second method, there is a method of using a ratio of signal strengths received from two microphones as indices when determining in which direction of left and right a sound source exists for an array including two microphones, or when separating sounds from signals including a left sound and a right sound mixed therein. This method is based on a phenomenon that a microphone located on a side where a sound source is present receives a larger sound than a microphone on an opposite side. In “Speech segregation based on sound localization” by N. Roman, D. Wang, and G. Brown, J. Acoust. Soc. Am., vol. 114, No. 4, pp. 2236-2252, 2003, a method of separating a sound source using this principle is presented.
The method on the basis of the signal strength ratio as described above is predicated on the fact that the sensitivities of the left and right microphones are the same.
Actually, however, the sensitivities of the microphones vary, and changes with time are not small, so that it is difficult to keep the same sensitivities all the time.
Therefore, there arises a problem of deterioration of performance in sound direction estimation and a sound source separation due to the variations in power ratio.